Sure there are nice free Udemy courses as well. Take a look at this one for example (Introduction to Graphic Design). But if you are into Usability, User Experience and Interaction Design there is one man you simply have to know: Dr. David Travis.

David Travis has authored and recorded the following fantastic Udemy courses:

Learn the basics of design theory to become a real graphic design master. Design goes way beyond software and there is a lot of theory behind it. In this introductory course, you'll learn the basics needed to create amazing and real design.